or am I just being thick?
no, just behind the times, using outdated software.
So does Photoshop 7 or CS have Pantone 871 then?
yes it does, but as with all pantone colors, it a designation of a greyacale, so you can pick any color and rename it to whatever you like. The separation is what’s important . The name just tells where that data goes at the time of separation.
Pantone 871c (an old-gold metallic) has a Lab value of 49, 3, 26
This translates on my computer with my color settings as 127 R, 113 G, 76 B.
You can choose that color from the picker for the swatch in your spot color channel if you need a representation of how your spot color will look on screen or in an inkjet print.